(1) Field of the Invention
This invention relates generally to printing apparatus and to apparatus for controlling the movement of a printing head; more particularly, the invention relates to a novel clutch or braking mechanism, and to use of the same in a data printer.
(2) Prior Art
In prior printers the limits of lateral print head movement were usually defined by setting left and right margin stops at the desired positions. Typically, such margin stops are mechanical elements which extend outward from the frame or other rigid portion of the data printer into the lateral travel path of the print head, and which are sufficiently rigid to withstand the entire force applied by a laterally moving print head, thereby stopping the print head by physically blocking it. When a print head is being laterally driven, the force needed to stop the print head in this manner can often be in the range of 50 to 60 pounds. Clearly, designing and constructing adjustable margin stops to withstand such magnitudes of force very materially increases the cost of the printer. Further, operator error in setting the margin stops may permit the print head to move off the printing surface and result in damage to the print head. Attempts at using various electronic control means to control the lateral travel of the print head add complexity and increase expense of fabrication of a data printer, and these are relatively susceptible to operator error and malfunction.
The damage to the print head can be particularly severe when the printing surface is relatively thick (e.g., multi-layered) and the print head is a wire matrix type. Although printing off the edge of the printing surface onto the platen may cause some damage, a more serious problem arises when returning the print head onto the printing surface to print the next line. For example, after the print head has been permitted to print beyond the edge of the printing surface, moving the print head from a position off the printing surface to a position on the printing surface with the needles extended can bend the needles, thereby destroying the head. This can happen either during initial set-up of the machine or when there are incorrect margin settings. These are some of the problems this invention overcomes.